1. Field of the Invention
This invention relates to the field of artificial joint prostheses and, in particular, to an improved instrument for machining a precise cavity in bone for receiving a prosthesis.
2. Description of the Prior Art
Noiles, U.S. Pat. No. 4,790,852, (hereinafter the "'852 patent"), describes a hip joint prosthesis employing a sleeve having an external configuration designed to mate with the internal configuration of the hard bone at the location where the prosthesis is implanted. A similar overall prosthesis geometry is shown in U.S. Pat. No. 5,002,578.
FIGS. 1 and 2 hereof show the sleeve of the '852 patent. As shown therein, sleeve 13 has a conical portion 6 centered on the sleeve's longitudinal axis 36 and a triangularly-shaped projection 8 which is offset from and extends away from the longitudinal axis. As shown in FIG. 2, the triangularly-shaped projection can be varied to make a set of sleeves of various sizes to fit different geometries in the bone.
The combination of a cone and a triangle can be precisely cut in the patient's bone using, for example, the instrument and technique shown in FIGS. 4-7 of the '852 patent. FIG. 7 of the '852 patent is repeated as FIG. 3 hereof. As shown therein, instrument 66 includes shaft 62 for aligning the instrument with the center of the bone and cutter 64 for cutting the cavity for the prosthesis' triangular projection. The triangular cavity is formed by moving the entire instrument 66 into the bone in the direction shown by arrow 70.
Although the instrument of the '852 patent has worked successfully in practice, it has suffered from the limitation that it does not define a positional relationship between the cone portion and the triangular portion of the prosthesis cavity. That is, the size of the triangular part of the cavity formed by the instrument depends upon how far the instrument is advanced into the bone by the surgeon. In practice, this problem has been addressed by having the surgeon select a sleeve having a triangular projection which most closely matches the size of the cut cavity. That is, in terms of FIG. 2, the surgeon selects a sleeve having the "A", "C", or "E" projection depending upon the size of the cavity he or she has cut. See the '852 patent at column 6, line 62, to column 7, line 4.
In implanting a prosthetic joint, a surgeon may prefer to select prosthesis component sizes prior to surgery. For the reasons described above, this is difficult to do with the instrument of the '852 patent. Specifically, it is difficult for a surgeon to select the size of the prosthesis' projection in advance since that size will depend upon how far the '852 instrument is advanced into the bone. That is, because the size of the triangular cavity is completely dependent on the depth to which the instrument is moved into the bone and because there is no reference as to that depth, the surgeon cannot cut a triangular cavity of a pre-selected size.
Another instrument for preparing a similar type of cavity has been developed in which the cutter for the triangular projection is pivoted about its distal end. Reference is made to Corin product brochure entitled "The Freeman Total Hip System," Corin Medical Limited, Gloucestershire, England, 1985. This approach can allow the surgeon to select the size of the triangular projection in advance. However, this device suffers from the disadvantage that a separate instrument is needed for each different projection size. That is, in terms of FIG. 2, three instruments would be needed to cut cavities for the "A", "C", and "E" projections
A pivoting instrument for cutting a cavity in the proximal end of the femur is shown in U.S. Pat. No. 4,777,942. This instrument cuts a cavity whose margin is an arc, rather than a straight line as needed to form a triangular-shaped cavity.